1. Field of the Invention
The invention relates generally to a technique for sensing minute disturbances such as shear waves, pressure changes, acoustic signals and the like and more particularly, but not by way of limitation, it relates to method and apparatus using linearly polarized light waves to derive an electric field vector indication of the change in length of a single-mode polarization-preserving fiber-optic cable.
2. Description of the Prior Art
The prior art includes various types of disturbance sensing systems that utilize fiber-optic sensing elements and cables of one type or another. Such systems of recent development use fiber-optic sensors to provide an indication of the change in length of a single-mode fiber cable that is influenced by the environment to exhibit a change in the phase of a light travelling through it. Light leaving the end of the fiber cable is then sensed for combination with light coming from a reference fiber cable that was isolated from the environment. The combination of the two light waves will then result in a high or low value of light due to constructive or destructive interference, and the resulting light level is related to the pressure that is exerted on the fiber-optic cable by the environment. U.S. Pat. No. 4,420,260 provides a representative teaching of the general class of phase-change sensing devices. These types of devices do not have the capability of accurately determining the frequency and amplitude of the sensed disturbance.